Fluorine gas or NF3 gas is obtained by using a fluoride-containing molten salt such as KF.2HF or NH4.2HF as an electrolyte and electrolyzing it.
As an electrolytic cell for electrolytic synthesis of the fluorine-containing material using the fluoride-containing molten salt as the electrolyte, there is used a box-shaped electrolytic cell partitioned into an anode chamber and a cathode chamber with a partition wall. Lower portions of electrodes are immersed in the molten salt, and these electrodes are connected to feeder bus bars in the electrolytic cell, thereby performing electrolysis. An electrode reaction proceeds at electrode portions immersed in the molten salt.
The HF vapor pressure of the fluoride-containing molten salt used as the electrolyte is high, so that an upper portion of the electrolytic cell which is not filled with the molten salt is filled with HF and fluorine gas or NF3 gas as a product for the anode side, and HF and hydrogen gas for the cathode side.
Corrosiveness of the fluoride-containing molten salt itself is very high, and the fluorine gas and the NF3 gas are also very high in corrosiveness and reactivity. Accordingly, for the electrode, particularly the anode, not only high catalytic activity to the desired electrode reaction is required at the portion immersed in the molten salt, at which the electrode reaction proceeds, but also reaction activity with the fluoride-containing molten salt and the fluorine gas or NF3 gas generated must be low. On the other hand, at an upper portion not immersed in the molten salt, anti-corrosiveness to HF and the fluorine gas or NF3 gas must be high, and reactivity to these must be low.
In industrial electrolysis, a carbon electrode or a nickel electrode has hitherto been used as an anode in many cases, and iron or nickel has been used as a cathode. The carbon electrode which has been practically used as an anode does not have sufficiently high anti-corrosiveness and low reactivity to the molten salt and the filled gas, and the nickel electrode also does not have sufficiently high anti-corrosiveness and low reactivity to the molten salt.
At the portion immersed in the molten salt, at which the electrode reaction proceeds, the carbon electrode reacts with the fluorine gas generated or a fluorine radical generated in a fluorine gas generation process to form graphite fluoride, thereby coming into a non-conductible state called an anode effect. Further, at a non-immersed portion, HF or the fluorine gas enters the inside of the electrode to cause electrode breakage to occur at a joint with the feeder bus bar and the like.
Accordingly, in conventional methods, in order to prevent entrance of HF or the fluorine gas and to inhibit the electrode breakage, it has been performed that the joint with the feeder bus bar is coated with nickel by a plating method or a thermal spraying method (for example, see patent document 1 and patent document 2).
Further, in the nickel electrode, the electrode breakage observed in the carbon electrode does not occur, but severe consumption occurs at the portion immersed in the molten salt.
Furthermore, as an electrolytic synthesis method of this kind, there has been proposed a conductive diamond electrode in which the anode effect observed in the carbon electrode and the electrode consumption observed in the nickel electrode do not occur and in which a conductive carbonaceous material showing high catalytic activity to the desired electrode reaction is used as a substrate (patent document 3).
In general, in industrial electrolytic synthesis of the fluorine gas or NF3 gas using a fluoride-containing molten salt, a carbon electrode or a nickel electrode of about 300×1,000 mm has been used. Also when the conductive diamond electrode is used, a size of about 300×1,000 mm is necessary. The conductive diamond electrode is produced by forming a conductive diamond film on an electrode substrate by a gas-phase synthesis method such as a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method. In an apparatus used widely, the size of the substrate applicable is approximately 300×300 mm or less, and it is difficult to produce an electrode having a size used in industrial electrolytic synthesis.
Only in a hot filament CVD method, one of the CVD method, an apparatus applicable to this size is present. However, even in this apparatus, it is difficult to form a uniform conductive diamond film to 300×1,000 mm, resulting in an expensive price. Further, also as for a hot filament CVD apparatus, a general-purpose type targets at approximately 300×300 mm or less.
When the fluorine gas or NF3 gas is synthesized using the conductive diamond electrode, a place requiring the conductive diamond film is only the portion to be immersed in the molten salt, at which the electrode reaction proceeds. However, in the above-mentioned CVD method or PVD method, it is necessary to insert the whole substrate into a reaction vessel, which inhibits an improvement in productivity and causes an increase in production cost.
The conductive diamond electrode is an excellent material exhibiting high catalytic activity and anti-corrosiveness. However, HF or the fluorine gas can not be prevented from entering the non-immersed portion, so that the problem of electrode breakage has not been solved yet.
In order to solve the problem of electrode breakage, it is necessary to coat a joint with a feeder bus bar with nickel, similarly to the carbon electrode. In order to coat the joint with nickel, the conductive diamond film once formed is required to be separated, which necessitates a complicated operation. A method of coating the joint with nickel before the conductive diamond layer is formed is impractical, because coated nickel deteriorates in a process of forming the conductive diamond layer.
Even when the conductive diamond electrode in which the joint with the feeder bus bar is coated with nickel is used, a process leading to electrode breakage (deterioration mode) is different from deterioration mode of an electrode catalyst immersed in the molten salt. Accordingly, the times taken for both to lead to deterioration are different from each other. Even when either of them is deteriorated, the electrode is required to be changed. It is difficult and useless to design so as to equalize the times taken for both to lead to deterioration, and it is desired that a portion not deteriorated can be reused.                Patent Document 1: JP-A-2000-313981        Patent Document 2: JP-A-60-221591        Patent Document 3: JP-A-2006-249557        